At most risk are bunker and stack silos, which have a much larger feeding face, on most farms entirely open to the elements. The almost daily rains we’ve been having throughout much of Northern dairy country can really make it tough to weigh out the silage in TMRs.

Relying on a silage DM test during a dry spell (remember those?), and then mixing this silage by weight right after a big rain, can result in serious underfeeding of the silage. Of course that’s because you’re feeding a lot of rainwater with the silage, and the face of a bunker or stack silo can really absorb the “liquid sunshine.”

In some cases, it would be better to feed heavily rained-on silage by volume rather than by weight: For instance, if you’ve been putting two front end loader buckets of corn silage into your TMR mixer (and know how much this has been weighing), you may be better off to feed two buckets of rained-on corn silage without paying attention to how much it weighs. 
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—Excerpts from Miner Institute Farm Report, May 2011